feat(prompt): universal task-completion guidance + local Python toolchain probe (#34340)

* fix(codex): surface error code in Responses 'failed' status errors

When a Codex Responses turn ends with status=failed, the response carries
the failure details under `response.error` as
`{code, message, param, ...}`. The previous extractor pulled only
`message`, so users seeing a rate-limit failure got a bare "Slow down"
string indistinguishable from a generic stream truncation; an
internal_error with empty message degraded to a dict dump
("{'code': 'internal_error', 'message': ''}").

Extract a `_format_responses_error()` helper that:
- prefixes `code` when both code and message are present
  (e.g. 'rate_limit_exceeded: Slow down')
- falls back to the bare `code` when message is empty
- accepts both dict and attribute-style payloads (SDK and JSON-RPC paths)
- preserves the prior status-only fallback when no error payload exists

Apply the same helper at the sibling site in
`codex_app_server_session.run_turn()` so codex-CLI subprocess turn
failures get the same treatment.

Tests:
- 8 new unit tests for `_format_responses_error` covering both shapes,
  empty/missing fields, non-string fields, and the status-only fallback.
- 2 regression tests on `_normalize_codex_response` for failed status
  with and without a code, asserting the exact RuntimeError message.
- All 3603 tests in tests/agent/ pass.

Adapted from anomalyco/opencode#28757.

* feat(prompt): universal task-completion guidance + local Python toolchain probe

Two cross-model failure modes get a single-line answer in the cached
system prompt. Both gated by config (default on), both add zero overhead
when not needed, both verified via real AIAgent prompt builds.

## What changed

`TASK_COMPLETION_GUIDANCE` — short prompt block applied to ALL models.
Targets two failure modes observed on a real Sarasota real-estate build
task: (1) Opus stopped after writing an 85-byte stub and gave a prose
response with finish_reason=stop on call #3 of 90; (2) DeepSeek pushed
through a PEP-668 wall, then returned fabricated listings instead of
admitting the blocker. Both behaviors are model-family-agnostic, so the
guidance lives outside the existing tool_use_enforcement gate (~192
tokens, paid once per session via prefix cache).

`tools/env_probe.py` — local Python toolchain probe. Detects
python3/pip/uv/PEP-668 state and emits ONE short line in the system
prompt when something is non-default. Emits NOTHING when the env is
clean (zero token cost for normal users). Skipped entirely for remote
terminal backends (docker/modal/ssh) — they have their own probe.

Example output on a broken environment (the actual case):

    Python toolchain: python3=3.11.15 (no pip module),
    python=missing (use python3), pip→python3.12 (mismatch),
    PEP 668=yes (use venv or uv).

## Config

Both flags live under `agent.` in config.yaml, default True:

    agent:
      task_completion_guidance: true   # universal "finish the job" block
      environment_probe: true          # local Python toolchain hints

Neither addition required a `_config_version` bump — deep-merge fills
defaults in for existing user configs.

## Validation

| Test surface | Result |
|---|---|
| tests/tools/test_env_probe.py | 10/10 pass (probe unit) |
| tests/run_agent/test_run_agent.py — new classes | 8/8 pass (integration) |
| TestToolUseEnforcementConfig | 17/17 pass (no regression) |
| TestBuildSystemPrompt | 9/9 pass (no regression) |
| TestInvalidateSystemPrompt | 2/2 pass (no regression) |
| tests/agent/test_prompt_builder.py | 124/124 pass (no regression) |
| tests/hermes_cli/ | 5662/5662 pass (config defaults) |
| E2E AIAgent build (broken env) | Both blocks present, 2,178 chars |
| E2E AIAgent build (clean env) | 771-char net overhead, env probe silent |

agent/transports/codex_app_server_session.py

@@ -31,6 +31,7 @@ import time
 from dataclasses import dataclass, field
 from typing import Any, Callable, Optional
 
+from agent.codex_responses_adapter import _format_responses_error
 from agent.redact import redact_sensitive_text
 from agent.transports.codex_app_server import (
     CodexAppServerClient,
@@ -581,7 +582,7 @@ class CodexAppServerSession:
                         (note.get("params") or {}).get("turn") or {}
                     ).get("error")
                     if err_obj:
-                        err_msg = err_obj.get("message") or str(err_obj)
+                        err_msg = _format_responses_error(err_obj, str(turn_status))
                         # If the turn failed for an auth/refresh reason,
                         # rewrite the error into a re-auth hint AND mark
                         # the session for retirement.